Methods are known for controlling the air/fuel ratio of an engine in accordance with the mixing ratio of alcohol in the fuel in a multi-fuel engine that operates using an alcohol/gasoline mixture, as disclosed in JP-S63-5131-A.
The engine described above is provided with an air intake duct for drawing in outside air, an injector provided in the air intake duct for injecting fuel, an exhaust duct through which exhaust gas passes, and an oxygen concentration sensor (hereinafter referred to as an “oxygen sensor”) for measuring the concentration of oxygen in the exhaust gas, the oxygen sensor being provided in the exhaust duct. A microcomputer computes the optimum air/fuel ratio according to the concentration of oxygen in the exhaust gas, and controls the engine. Specifically, when alcohol is mixed in with the fuel, an air/fuel ratio that conforms to the alcohol mixing ratio is automatically set, and the engine is controlled to a preferred state by feeding back and correcting the air/fuel ratio according to operating conditions.
In the technique described above, zirconia is used in the oxygen sensor, and when the oxygen sensor that uses zirconia reaches a temperature equal to or above a predetermined temperature, the oxygen concentration is measured with a predetermined accuracy of detection. In other words, the predetermined accuracy of detection cannot be obtained by the zirconia before the oxygen sensor has reached a predetermined temperature.
Fuel is usually refilled while the engine is stopped. In a vehicle provided with a multi-fuel engine, the person doing the refueling can freely choose to refill any amount of a particular type of fuel during refueling. The newly filled fuel therefore mixes with the remaining fuel in the fuel tank, and the mixing ratio is not easily detected.
Even when ethanol or gasoline has been refilled while the engine is stopped, the mixing ratio of the fuel that remains in the fuel duct is still the same as the mixing ratio prior to refueling. Therefore, when the engine is to be started, it is efficient to use a method in which the startup is controlled using a reference fuel injection rate map that was used immediately prior to the last engine stoppage.
Since the ethanol concentration can be detected based on learning control or the like of the ethanol concentration using the measured value of the oxygen sensor once the oxygen sensor reaches a predetermined temperature during consumption of the fuel that remains in the fuel duct, the fuel remaining in the fuel duct is consumed, a transition is made to the new fuel having a different ethanol concentration, and the engine is gradually transferred to a control procedure in which fuel is injected at an air/fuel ratio commensurate with the new ethanol concentration even when there are abrupt changes in the ethanol concentration.
In a cold start, however, the engine consumes the fuel remaining in the fuel duct and is switched to the newly filled fuel having an unknown mixing ratio before control is started by detection of the ethanol concentration, and a state can occur in which the air/fuel ratio cannot be coordinated with the new target ethanol concentration if the reference fuel injection rate map used is the one used immediately before engine stoppage. When the predetermined detection accuracy is not obtained, problems occur in that it is difficult to combust fuel at the preferred air/fuel ratio in the engine.